The invention relates to a process for producing a thin sheet of ultra-low-carbon steel for the manufacture of drawn products for packaging, such as cans, and a thin sheet obtained by the process.
In order to manufacture, by drawing, steel packaging products such as cans for foodstuffs or for drinks, blanks are used which are cut from thin sheets whose properties have been tailored to the drawing-type forming process.
The drawing processes used for manufacturing cans for preserved food or for drinks are generally drawing-redrawing (DRD) or drawing and wall ironing (DWI) processes.
In either case, it is known to use thin sheets of very-low-carbon or ultra-low-carbon (ULC) steel whose carbon content by weight is a few thousandths of a per cent and generally less than 8 thousandths of a per cent.
A process is known, for example from FR 95/02208, for producing a thin sheet intended for the manufacture of a can, of the drinks-can type, by drawing and wall ironing using a steel having the following composition by weight:
carbon  less than 0.008%,
manganese, between 0.10 and 0.30%,
nitrogen  less than 0.006%,
aluminum, between 0.01 and 0.06%,
phosphorus  less than 0.015%,
sulphur  less than 0.020%,
silicon  less than 0.020%,
at most 0.08% of one or more of the elements copper, nickel and chromium, the balance of the composition consisting of iron and inevitable impurities.
In general, in the case of the manufacture of cans by the drawing-redrawing (DRD) or drawing and wall ironing (DWI) processes, specific mechanical properties and drawability characteristics are required with regard to the thin sheets or to the blanks cut from these sheets which are subjected to the drawing operation.
In particular, the thin sheets must have a low tendency to form ears during drawing and must have very good properties for being able to be drawn by necking.
Good drawability is characterized by a high Lankford coefficient or normal anisotropy coefficient and by a plane anisotropy coefficient EC close to zero.
Furthermore, it is also sought to obtain a microstructure of the steel which is as homogeneous as possible over the width of the sheet and along its edges, so as to obtain homogeneous behaviour of the blanks while they are being drawn. In addition, a microstructure as close as possible to a microstructure containing homogeneous equiaxed grains is desired in the sheet intended for drawing.
Because the thickness of the metal packaging in the finished state may be very small (for example, less than 0.1 mm), it is also necessary to use a sheet free of defects such as inclusions, i.e. a material having the best possible inclusion cleanliness.
The thin steel sheets for manufacturing drawn packages are generally produced from an aluminum-killed vacuum-degassed steel, generally cast continuously in the form of a slab which is then hot rolled so as to obtain a hot-rolled strip which is then cold rolled in two steps separated by a recrystallization annealing step.
The second rolling operation, which is generally carried out on a skin-pass rolling mill, makes it possible to obtain a sheet having the final thickness of the product on which the drawing operation is carried out.
In the case of the manufacture of ultra-low-carbon steels, the steel produced in the metallurgical furnace is subject to vacuum degassing, generally with the injection of oxygen, and is aluminium killed before being cast in a continuous casting plant for producing a slab.
The slab is hot rolled at a temperature above the Ar3 point of the steel in order to obtain a hot-rolled sheet whose thickness is generally less than 3 mm.
Next, the hot-rolled sheet is cold rolled with a reduction ratio generally greater than 80% in order to obtain an intermediate cold-rolled sheet or blank which is then annealed at a temperature below the Ac1 point of the steel before the final skin-pass rolling, the reduction ratio of which depends on the intended application of the sheet.
Vacuum-degassed aluminium-killed ultra-low-carbon steel sheets have suitable characteristics with regard to their drawability, the homogeneity of the microstructure obtained after the manufacturing cycle, and the inclusion cleanliness.
However, the manufacture of novel packages of complex shapes with ever thinner walls requires ever higher properties to be obtained.
A process has been proposed in EP-0,521,808 for producing sheets intended for deep drawing, for example for the manufacture of cans by the DRD process from a converter-smelted steel containing at most 0.015% carbon and less than 0.040% aluminium. The process includes hot rolling. The hot-rolled sheet is coiled at a temperature above 650xc2x0 C., then cold rolled and finally annealed at a temperature below 700xc2x0 C. The need to coil at a temperature above 650xc2x0 C. leads to heterogeneities in the properties of the strip, in the transverse direction and between the ends and the core of the coil. In addition, coiling at a temperature above 650xc2x0 C. leads to a hot-rolled sheet structure which is not very favourable for obtaining a fine-grained cold-rolled sheet (ASTM index greater than 9).
U.S. Pat. No. 3,404,047 describes a process for manufacturing a sheet for deep drawing having a very low carbon content (C xe2x89xa60.004%). This very low carbon content is obtained by carrying out a decarburizing annealing operation on the sheet. Because of the annealing conditions (2 to 20 hours at 715xc2x0 C.), the grain index of the sheet is very low (6 to 7).
EP-0,659,889 describes a process for manufacturing a cold-rolled sheet containing a very small proportion of carbon (C xe2x89xa60.004%) and having a very low aluminium content (between 0.005 and 0.070%). The steel has a niobium content which is greater than 0.001% and which can be as much as 0.018%. Because of the presence of niobium, the recrystallization temperature of the steel, and therefore the temperature of the recrystallization annealing, is substantially higher than in niobium-free steels.
The object of the invention is to provide a process for producing a thin sheet of ultra-low-carbon steel for the manufacture of drawn packaging products, in which process:
a killed and vacuum-degassed steel containing, by weight: between 0.10 and 0.35% manganese, less than 0.006% nitrogen, less than 0.025% phosphorus, less than 0.020% sulphur, less than 0.020% silicon, at most 0.08% of one or more elements from among copper, nickel and chromium, as well as aluminium, the balance of the composition consisting of iron and inevitable impurities, is produced,
the steel is cast in the form of a slab,
the slab is hot rolled at a temperature above Ar3 in order to obtain a hot-rolled sheet,
the hot-rolled sheet is coiled,
the hot-rolled sheet is cold rolled into the form of an intermediate cold-rolled sheet,
the intermediate cold-rolled sheet is continuously annealed at a temperature below Ac1, and
the intermediate cold-rolled sheet is rerolled to a final sheet thickness for drawing, the process according to the invention making it possible to substantially improve the drawability, the inclusion cleanliness and the microstructural homogeneity of the sheet for drawing.
To this end, the steel is produced so as to contain at most 0.006% carbon by weight and 0.010% aluminium by weight and the hot-rolled sheet is coiled at a temperature below 620xc2x0 C. and preferably between 530xc2x0 C., and 570xc2x0 C.
The invention also relates to a production process in which the steel is killed by bringing an unkilled steel obtained by smelting in a m tallurgical furnace into contact with a slab containing, in particular, aluminium and alumina Al2O3.
The invention also relates to a production process in which the steel is cast in the form of a slab in an inert-gas continuous casting plant.
Finally, the invention also relates to a thin sheet having a homogeneous equiaxed-grain structure with a low inclusion content and having very good drawability characteristics, made of an ultra-low-carbon steel containing less than 0.010% aluminium.